[llvm] [MC, X86] emitInstruction: remove virtual function calls due to Intel JCC Erratum (PR #96835)

[Fangrui Song via llvm-commits]

MaskRay wrote:

The overhead introduced by these fragment-related features are much higher than I expected.

* Intel JCC Erratum mitigation
* prefix padding for alignment directives ( https://reviews.llvm.org/D75300)
* allowEnhancedRelaxation (making all instructions FT_Relaxable so that we can add prefixes to them), https://reviews.llvm.org/D76286

https://llvm-compile-time-tracker.com/compare.php?from=ff0cbecb68bd28f6131894fbb037e063e8da6bab&to=d77ac81e93e5e2df5275b687b53049d9acfe1357&stat=instructions:u claims another 0.10% saving from stage2-O0-g: instructions:u.


https://github.com/llvm/llvm-project/pull/96835